Planetary Boundaries

Recognizing that the Planetary Boundaries and associated control variables, boundary determinations and assessments are somewhat controversial and imperfectly assessed, the PB concept is nonetheless valuable for conceptualizing global change in the Anthropocene. It has helped frame the discussion regarding human modification of the global environment and helped identify why unmitigated change in the 9 broad categories may threaten the manifold ecosystem services upon which civilization depends.

Furthermore, the Planetary Boundary phrase, like the Anthropocene, have come into wider usage, well beyond the confines of the scientific literature. I believe they can help students and the broader public gain a deeper appreciation that planetary change extends well beyond climate change.

Classic Planetary Boundary Image illustrating 9 PBs and current status

Planetary boundaries identify key physical conditions that are needed to maintain the integrity of the biosphere and the Earth system. This approach identifies nine environmental areas where transgressions could lead to shifts that could irreversibly move the biosphere out of the stable conditions which characterized the Holocene. They are: 

  • Stable climate 
  • Intact biodiversity 
  • Sufficient nutrients (but no overload) 
  • Protective ozone layer 
  • Absence of pollutants 
  • Clean and sufficient fresh water 
  • Stable and fertile soils 
  • Absence of acidification in both water and soils 
  • Source: http://

Current status of the control values for seven of nine planetary boundaries.  Control values are bracketed between the Planetary Boundary (PB) lower and upper range (yellow zone of uncertainty).

Image Adapted from Steffen, W.; Richardson, K.; Rockström, J.; Cornell, S.E.; Fetzer, I.; Bennett, E.M.; Biggs, R.; Carpenter, S.R.; de Vries, W.; de Wit, C.A.; et al. Planetary boundaries: Guiding human development on a changing planet. Science 2015, 347, 1259855.

Explanation of the PB Boundaries

Green zone is the “safe operating space for humanity”

Yellow is zone of uncertainty

Red zone is area of high risk and uncertainty

Grey indicates planetary boundaries for which the control values are unknown.

Earth System Process (italics) and Control Variable

            Climate change:  Atmospheric carbon dioxide concentration

            Ocean Acidification:  Percentage relative to carbonate ion concentration with respect to aragonite of the pre-industrial state surface ocean.  Current value near the upper limit of uncertainty

            Stratospheric ozone:  Ozone concentration in Dobson Units (DU)

            Biogeochemical Flows:  Nitrogen—Industrial and intentional biological fixation of N

            Phosphorus—P flow from freshwater systems into the ocean

            Freshwater Use:  Maximum amount of consumptive blue water use (km3/yr).  Blue water (rivers, lakes, reservoirs, and renewable groundwater stores)

            Land System Changes:  Amount of remaining forest cover (tropical, boreal, temperate) as a percentage of original forest cover

            Biosphere Integrity (There are two control variables)

                        —genetic diversity (extinction rate given as extinctions per million species-year)

                        –Functional Diversity, measured as a percentage of biodiversity intactness (interim control values) for both major terrestrial biomes and major marine ecosystems, such as coral reefs.

            Novel Entities:  No control value; includes new substances (e.g., plastics,chemicals,persistent organic pollutants, GMO’s, heavy metals, CFC’s, etc. 

Atmospheric Aerosol Loading:  Aerosol optical depth (AOD); but no global value exists.  Aerosols impact human health and climate.